In conventional semiconductor technology based on inorganic semiconductors, a period of several months is generally required in order to fabricate first patterns of the corresponding chips from a circuit design. These comparatively long periods of time are caused, in part, by the multiplicity of very complicated production steps comprising a wide variety of methods such as photolithography, deposition etching processes and the like that are involved when progressing through the fabrication of the microchip.
For specific applications, conductive organic polymers are appropriate in the long term as an alternative to the inorganic semiconductor materials that have been used heretofore. Organic layer systems can be applied to a substrate in a relatively uncomplicated manner by means of suitable printing techniques. In contrast to microelectronic components based on inorganic semiconductors, structures made of organic conductive materials can be fabricated comparatively cost-effectively.
However, the task of patterning the conductive material in accordance with the circuit logic to be obtained is still required for organic layer systems. If polymer layers are deposited onto a substrate, there are in principle the possibilities of either patterning the polymer layer after deposition, defining individual regions on the substrate in a targeted manner prior to deposition, or controlling the deposition of the polymer onto the substrate in a defined manner. The last-mentioned possibility affords the advantage of saving material costs and complicated cleaning or recovery processes.
If the polymer layer is intended to be patterned only after deposition, then it is possible, by way of example, to incorporate suitable photoactive components into the polymer and to generate a polymer having photoactive properties in this way. Through selective exposure, it is then possible, by way of example, to alter the solubility of the polymer in a developer in a targeted manner.
As an alternative, it is also possible to apply a photoresist layer to the polymer layer, which photoresist layer is initially selectively exposed with the aid of a photomask in order to prepare for a patterning. After the development of the photoresist layer, a mask is obtained, and the uncovered sections of the polymer are removed in a subsequent etching step, in most cases a plasma process. Finally, the photomask also has to be removed, e.g. by means of a suitable solvent.
A considerable disadvantage of this method is the need for a costly photolithographic process step. Besides high material and apparatus costs for the provision of the photomasks, photomask exposers, photoresists, developer solutions and solvents, in some instances special wastes that can only be disposed of with a high outlay also arise during production. Moreover, the technique of photolithography only permits a limited number of substrate elements to be patterned in a single production cycle, corresponding outage times for preparation and follow-up measures likewise having to be taken into consideration, which lead to a low process throughput.
In addition to optical methods for selective exposure using photomasks, it is also possible to use mechanical methods for patterning. Thus, by way of example, the screen printing method affords the possibility of applying a patterned resist layer to the polymer. Analogously to the optical methods, this method likewise requires a structure transfer process for transferring the resist layer onto the substrate in an etching process and the subsequent removal of the resist. In comparison with the optical methods, however, the structure resolution that can be achieved is only approximately 200 μm and can therefore only be used for producing relatively coarse structures. It is thus sufficient for imaging large-area interconnects and electrodes but has to be replaced by higher-resolution methods in the case of finer structures, such as large scale integrated circuits.
Inkjet printing is also a mechanical method for producing defined structures in polymer layers. In this case, the polymer is sprayed onto the substrate surface as a solution in the form of small droplets. This requires the polymer either itself to be present in a printable constitution or to be correspondingly prepared by admixture of suitable solvents and additives. The additives and solvents should rapidly evaporate after application in order to prevent the layer structure from running together.
A method is disclosed in Sirringhaus, H, Kawese, T. Friend, R.H.: High-Resolution Ink-Jet Printing of All-Polymer Transistor Circuits, in MRS Bulletin (July 2001), in which a template is used to define the regions to be printed on the substrate surface. The template comprises fine polyimide structures which are not wetted by the polymer printing ink and thus serve as a structure base in the subsequent inkjet printing.
However, the inkjet method has a series of disadvantages. The polyimide structures initially have to be patterned by photolithographic methods, resulting in high costs being incurred. In addition, dispensing with the use of polyimide structures leads to a reduction of the resolution as a result of the printing structure running together. As a further disadvantage, the inkjet method permits only a line-by-line structure construction, thereby causing a low throughput rate and hence high process costs.
Consequently, while the high resolutions of photolithographic patterning techniques are associated with high costs, the less expensive screen printing or inkjet methods permit only a low resolution.